Publishing post causes 404 if link included

Here’s a strange little bug: if I publish or save a blog post that includes a link in the content, it takes me to a 404 page (specifically wp-admin/post.php though the file DOES exist); if there’s no link (or the URL has not been hyperlinked, just pasted as text) then the post updates as usual. I’ve tried it with all plugins off, and with nothing but a default theme. I’ve also tried to flush permalinks. No use.